This application claims the priority of Swiss patent application 2205/02, filed Dec. 23, 2002, the disclosure of which is incorporated herein by reference in its entirety.
The invention relates to a device for measuring the flow of a fluid, i.e. of a gas or a liquid, as well as to a bypass for such a device.
A device of this type is disclosed in WO 01/98736. It comprises a primary duct and a bypass. A flow sensor is arranged in the bypass. An advantage of such an arrangement is the fact that the flow rate to be measured in the bypass is smaller than in the primary duct, and it is therefore possible to measure higher flow rates.
While devices of this type have a wider measuring range, it has been found to be disadvantageous that the conversion of the flow in the bypass to the actual flow in the primary duct is prone to systematic errors.
Hence, it is a general object of the invention to provide a device for measuring the flow of a fluid in a bypass that provides higher accuracy.
Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the device is manifested by the features that it comprises a primary duct, a bypass parallel to said primary duct, said bypass having a first section and a second section, a flow sensor arranged at said bypass, wherein the first section of the bypass generates a pressure drop xcex94p1=c1xc2x7vb+c2xc2x7vb2 with c2xe2x89xa00, wherein vb is the flow rate in the first section, and wherein the second section of the bypass comprises at least two secondary ducts arranged in parallel, wherein the secondary ducts are arranged in series to the first section.
In another aspect of the invention, a bypass for such a device is provided. The bypass is designed for being connected to a primary duct for measuring a flow of a fluid therein. The bypass comprises a flow sensor, a first section generating a pressure drop xcex94p1=c1xc2x7vb+c2xc2x7vb2 with c2xe2x89xa00, wherein vb is the flow rate in the first section, and a second section arranged in series to the second section, wherein the second section comprises at least two secondary ducts arranged in parallel, wherein the secondary ducts are arranged in series to the first section.
As the analysis below shows, it is advantageous to design the bypass in such a way that that the ratio of the flows in bypass and primary duct is substantially constant. This can e.g. be achieved by arranging at least one steplike, discontinuous change in diameter in the bypass, e.g. by using a baffle plate with a hole therein. In order to have comparatively strong turbulent contributions to the flow resistance, the diameter of the part that gives rise to the turbulence can be chosen to be very small. Such structures are, however, difficult to manufacture and suffer from problems due to clogging in operation.
Hence, the diameter of the structure causing the turbulence should not be too small. Hence, in order to achieve the desired turbulence, the flow in the bypass should still be fairly large, which in turn would lead to exceedingly large flow rates at the sensor. Therefore, according to the invention, a first part of the bypass is designed such that it shows a distinct dependence of the flow resistance and the pressure drop from the square of the flow rate vb. In a second section of the bypass, at least two parallel secondary ducts are provided, which are arranged in series to the first section. The flow sensor measures the flow in one of the secondary ducts.
By dividing the fluid in the second section into two or more secondary ducts and by an appropriate selection of the ratio of the diameters of the secondary ducts, the flow rate or flow velocity to be monitored by the flow sensor can be comparatively small. In the first section, however, the flow rate is the sum of all flow rates in the secondary ducts and can therefore be higher and give rise to strong turbulent effects.
Since two or more secondary ducts are used, the system designer can use additional design parameters for optimizing the flow conditions for the desired measuring range.
Dividing the second section into several secondary ducts has the further advantage that the diameter of the secondary duct where the flow sensor is arranged can be comparatively small, which improves the laminarity of the flow at the sensor and therefore the accuracy of measured results.
The described embodiment can be used for measuring gases or liquids.